1. Field of the Invention
The present invention relates to a galvannealed steel sheet, suitable for producing body parts of automobiles. The invention is also concerned with a method of producing it.
2. Description of the Related Art
Galvannealed Steel sheets exhibit superior corrosion resistance and, hence, are broadly used as the material of automobile body parts. Materials of automobile body parts are required to have corrosion resistance property as well as other characteristics such as press-workability, resistance to peeling of plating layer during press work, and spot-weldability.
In general, a continuous hot dip galvannealing process does not allowed a lengthy time period for heating and soaking. Therefore, in the production of plates steel sheets for automobile body parts which are required to have high press-workability, steel sheets having very low carbon content, which generally exhibit excellent press-workability with short heating and annealing, are used as the base materials, as disclosed, for example, in Japanese Patent Publication No. 60-48571.
However, galvannealed steel sheets having very low carbon content, exhibit inferior spot-weldability characteristics for reasons which will be explained later. The result is that the efficiency of the automobile body assembly process is seriously impaired.
In order to obviate these problems, it has been proposed to increase the Fe content in the plating layer or to coat the surface of the plating layer with a ferrous alloy. The first-mentioned method, however, is disadvantageous in that exfoliation or peeling of the plating layer tends to occur when the Fe content is increased to a level which provides the desired level of spot-weldability. On the other hand, the second-mentioned method causes the production cost to be raised seriously and reduces corrosion resistance after painting.